Since the early 1990's, communication systems based on chaotic carriers have been proposed. Chaotic signals can be generated with very simple circuitry and are characterized by a wide bandwidth. By using a chaotic carrier to spread the digital signal over a wide frequency band, the resulting system inherits the benefits of spread-spectrum communications such as mitigation of multipath fading and low probability of detection.
Communication systems based on chaos can be broadly categorized into two groups. In the first group, chaotic signals carrying the information must be synchronously reproduced at the receiver in order to recover the information, Many synchronization techniques have been proposed and studied in the literature, for example, U.S. Pat. Nos. 6,363,153 and 6,331,974. Communication systems based on synchronized chaos have a high security because identical chaotic circuits are required at the transmitters and receivers. However, synchronization techniques are only stable under a very low noise environment. When the noise level is increased to a practical level, synchronization will fail and the communication systems no longer function properly.
In the second group of chaos-based communication systems, the chaotic carriers need not be regenerated at the receiving end. The receivers determine the transmitted information based only on the incoming chaotic signals. An example of such systems is the differential chaos-shift-keying (DCSK) scheme, which is described in the original paper by Kolumbá´n et al. in 1996, (“Differential chaos shift keying: A robust coding for chaos communications” published in the 1996 Proceedings of International Specialist Workshop on Nonlinear Dynamics of Electronics Systems, pp. 87-92). In this system, each bit duration is divided into two equal time slots. In the first time slot, a reference chaotic signal is sent. Dependent upon the binary symbol being sent, the reference signal is either repeated or multiplied by “−1” and transmitted in the second time slot. The chaotic signal in the second time slot is known as the information-bearing chaotic signal because it carries the binary symbol being sent. However, because of the regular bit structure and the high correlation between the reference chaotic signal and the information-bearing chaotic signal, the bit frequency can be easily determined from the transmitted signal, jeopardizing the security of this system.
In addition, chaotic signals, being wideband, occupy a bandwidth much larger than what is required to transmit the information. Hence, more than one user should be able to transmit information in the same frequency band. Multiple access techniques based on the differential chaos-shift-keying scheme, as described in the papers by Kolumbán et al. in 1997, (“Multilevel differential chaos shift keying” published in the 1997 Proceedings of International Specialist Workshop on Nonlinear Dynamics of Electronics Systems, pp. 191-196) and by Lau et al. in 2002, (“A multiple access technique for differential chaos shift keying” published in IEEE Transactions on Circuits and Systems I, pp. 96-104), make use of different transmitted bit/frame structures for different users to minimize the interference between users. However, the similarity between the reference chaotic signal and the information-bearing chaotic signal remains. Hence, anyone with a simple correlator-type receiver can decode the signals easily.